J. Am. Chem. SOC. 1988, 110, 6612-6617
6612
Registry No.H+, 12408-02-5;NH, 13774-92-0; N-, 18851-77-9; Sc', 14336-93-7;Ti+, 14067-04-0;Y+, 14782-34-4;Zr', 14701-19-0;Nb', 18587-63-8;Lat, 14175-57-6;Ta', 20561-66-4;V', 14782-33-3;Fe', 14067-02-8;Sct-NH, 1 1 5858-87-2;Tit-NH, 115858-88-3;Yt-NH, 115858-89-4; Zr+-NH, 115858-90-7; Nb+-NH, 115858-91-8;Lat-NH,
115858-92-9;Ta+-NH, 115858-93-0;Vt-NH, 115858-94-1;Fe*-NH, 1 1 5858-95-2;V+-C,H5, 1 15858-96-3;V-N, 24646-85-3;Crt, 14067-03-9; c-C6H6,71-43-2; 02,7782-44-7;CzH4,74-85-1;propene, 115-07-1;amsec-butylamine, 13952-84-6;n-propylamine, 107-10-8; monia, 7664-41-7; pyridine, 110-86-1;diethylamine, 109-89-7;triethylamine, 121-44-8.
Surface-Enhanced Raman Study of the Effect of Electrode Potential and Solution pH upon the Interfacial Behavior of 4-Pyridinecarboxaldeh yde Mark R. Andersont and Dennis H. Evans*-* Contribution from the Department of Chemistry, University of Utah, Salt Lake City, Utah 841 12, and Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716. Received March 14, 1988. Revised Manuscript Received June 1 , 1988
Abstract: Surface-enhanced Raman scattering (SERS) spectra were obtained at a silver electrode in aqueous solutions at potentials from -0.2 to -0.6 V vs SCE and pH 1-7. The forms of 4-pyridinecarboxaldehyde that are adsorbed parallel those present in solution. In particular, the fraction of adsorbed species that is protonated at -0.2 V varies with pH in a manner almost identical with the fraction that is protonated in bulk solution. The protonated compound exists almost completely in the hydrated form (gem-diol) in the interface just as it does in solution. At a given pH, the fraction protonated on the surface increases as the potential is made more negative, a behavior attributed principally to enhanced adsorption of cationic species at negative potentials. In the presence of chloride, the fraction protonated decreases at the most negative potentials. This decrease is correlated with the desorption of chloride, suggesting that the pyridinium ion and chloride are adsorbed together. When the more strongly adsorbed bromide or iodide is used, the fraction protonated does not decrease at negative potentials.
A reactant molecule may exist in a variety of forms related to one another by acid-base reactions, complexation, tautomerization, etc. Frequently, one of these forms is more reactive than the others, causing the reaction to proceed via that particular species. A complete understanding of the reaction mechanism requires the identification of the reactive form, a task usually accomplished through a detailed evaluation of the reaction kinetics. When the reaction occurs at an interface (as in heterogeneous catalysis), it is highly desirable to have a very selective means of detecting adsorbed species while the reaction is occurring. The reduction of aldehydes is an example of such a reaction. In aqueous solutions many aldehydes exist in two forms, the free aldehyde and the hydrate (gem-diol), with the relative concentrations being governed by a hydration equilibrium constant, Kh. R C H O H,O = RCH(OH), Kh = [RCH(OH),]/[RCHO]
+
In the electrochemical reduction of such aldehydes, it is the free aldehyde that is the active form. In order for the gem-diol to be reduced, it must first dehydrate to form free aldehyde, which in turn is reduced at the electrode surface. The dehydration reaction, which is often the rate-limiting step, can occur in solution near the electrode or, possibly, on the electrode surface. The electrochemical reduction of 4-pyridinecarboxaldehydehas been the subject of intense study.'-' In acidic media over 90% of the aldehyde is hydrated,' but it can be rapidly and efficiently reduced to the alcohol, 4-pyridylcarbinoL6 The dehydration reaction has been shown to be a crucial feature of the reduction mechanism under a variety of solution condition^.'-^-^^^ Though the dehydration is normally considered to occur only in solution near the metal surface, a significant fraction may react o n the 'University of Utah. *University of Delaware.
0002-7863/88/1510-6612.%01.50/0
surface, a possibility enhanced by the strong adsorption of pyridine and its derivatives. In order to investigate the adsorption of the free and hydrated forms of 4-pyridinecarboxaldehyde, a selective and sensitive method is needed. Surface-enhanced Raman spectroscopy (SERS) is an attractive candidate. In fact, the original observations of S E R S were obtained with pyridine at a silver surface.*-1° As later investigations demonstrated, SERS is unusually powerful for in situ investigationof metalsolution interfaces. The sensitivity is excellent and the information content of the spectra is high, permitting resolution and identification of very similar surface species. In this paper we report the application of S E R S to the characterization of the various forms of 4-pyridinecarboxaldehyde adsorbed at silver surfaces as a function of solution p H and electrode potential. As will be seen, systematic investigation of the effects of these variables proved to be crucial in the detection of the adsorbed reactive free aldehyde, a species that was not (1) Laviron, E. Bull. Soc. Chim. Fr. 1961, 2325-2349. (2) Blizquez, M.; Camacho, L.; JimBnez, M.; Dominguez, M. J . Electroanal. Chem. Interfacial Electrochem. 1985, 189, 195-202. (3) Camacho, L.; Bllzquez, M.; JimBnez, M.; Dominguez, M. J . Electroanal. Chem. Interfacial Electrochem. 1984, 172, 173-179. (4) Rusling, J. F.; Zuman, P. J . Org. Chem. 1981, 46, 1906-1909. (5)Rusling, J. F.; Zuman, P. J . Electroanal. Chem. Interfacial Electrochem. 1983, 143* 283-290. (6) Nonaka, T.;Kato, T.; Fuchigami, T.; Sekine, T. Electrochim. Acta 1981, 26, 887-892. (7)Bhatti, M.; Brown, 0. R. J . Electroanal. Chem. Interfacial Electrochem. 1976, 68, 85-95. (8) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Chem. Phys. Lett. 1974, 26 163. (9)Jeanmaire, D. L.;Van Duyne, R. P. J . Electroanal. Chem. Interfacial Electrochem. 1977, 84, 1-20. (10)Albrecht, M. G.;Creighton, J. A. J . Am. Chem. Soc. 1977, 99, 5215-5217.
0 1988 American Chemical Society
Interfacial Behavior of 4-Pyridinecarboxaldehyde
J. Am. Chem.SOC.,Vol. 110,No. 20, 1988 6613
present a t the pH and potential used in the only previous S E R S investigation of 4-pyridinecarboxaldehyde,that of Bunding and Bell."
Experimental Section 4-Pyridinecarboxaldehyde (Aldrich Chemical Co.) was distilled at reduced pressure and stored under a nitrogen atmosphere prior to use. Water was triply distilled, and all other reagents were analytical reagent grade. Acetate buffers were used for pH values between 3.5 and 5.5 while phosphate buffers were used for pH 5.5-7. The pH of solutions below pH 3.5 was adjusted with HCI. All solutions contained 0.10 M KCI (or 0.10 M KBr or KI in certain experiments as specified), and the ionic strength of the buffers was adjusted to 0.90 M by addition of potassium nitrate. The SERS cell was built following the design of Brandt.12 The polycrystalline silver electrode was a disk (3-mm radius) prepared by press-fitting a cylinder of silver into one end of a 9.5-mm-diameterTeflon rod through which a 6-mm-diameter concentric hole had been drilled. Electrical contact was made via a copper wire soldered to the silver. Prior to the experiment, the silver electrode was polished with 5-, 0.3-, and 0.05-Km alumina. After polishing, the electrode surface was rinsed with copious amounts of triply distilled water followed by sonication in triply distilled water. The SERS cell was then assembled and filled with an analyte solution, which had been previously purged with nitrogen. The silver electrode was then subjected to an oxidation-reduction cycle (ORC), viz., oxidation at 0.20 V for 10 s followed by reduction of the generated silver chloride at -0.30 V until the current nearly ceased. When KBr was used in the electrolyte, the potentials were +0.20 and -0.60 V, and with KI -0.20 and -0.80 V were selected. About 100 mC/cm2 was passed in the oxidation and reduction steps. For convenience, all spectra reported were obtained after an ORC with the solute present. When the solute was added after the ORC, very similar spectra were recorded. The spectra were taken with the 488.0-nm line of a Spectra Physics Model 164-00 argon ion laser with an incident power of 100 mW at the electrode surface. The laser light was focused to a line image at the electrode by a cylindrical lens. The scattered light was focused onto the entrance slit (2.0-cm-' resolution) of a Spex Model 1401 double monochromator, and detection was by photon counting with an RCA Model C31034-02 photomultiplier tube. Data collection was performed with a microcomputer system. The intensity of the SERS spectra varied considerably with changes in conditions. All spectra have been plotted with an arbitrary scale for the ordinate. Sharp lines at 223,738, 1010, 1055, and 1578 cm-' that appear in various spectra are from scattered argon ion emission. The electrode potential was controlled by a Princeton Applied Research EG&G (PAR) Model 173 potentiostat/galvanotat and is referenced to a saturated calomel electrode. A PAR Model 276 current-tovoltage converter allowed monitoring of current during the ORC and SERS experiments. Positive-feedback iR compensation was employed though the ohmic losses were low in the SERS experiments due to the inherently low currents.
Results and Discussion Detection of Adsorbed Free Aldehyde. S E R S spectra of 0.050 M 4-pyridinecarboxaldehydein 0.10 M KCl at various electrode potentials are shown in Figure 1. As the electrode potential is made more negative, the spectrum undergoes dramatic changes. Features at 1070,1250, 1325, 1420, 1470, 1490, 1535,and 1560 cm-l appear and become more intense as the potential is made more negative. Additionally, a small feature at 1640 an-'appears at about -0.40 V, and this band also increases in intensity at more negative potentials. As will be shown later, the band at 1640 a n - I is due to the protonated pyridine and will be very valuable in characterizing the adsorbed species. Significantly, at -0.20 V there is a small but easily detectable band corresponding to the carbonyl stretching frequency at 1710 cm-I. It is also potential-dependent, becoming weaker and finally disappearing altogether at about -0.5 V. This confirms the result of Bunding and Bell" who reported no carbonyl band because they obtained their spectra at a single potential, -0.60 V. Thus, the reducible form of 4-pyridinecarboxaldehyde,the free aldehyde, is adsorbed at the silver electrode as long as the potential ( I 1) Bunding, K. A.; Bell, M.I. Surf. Sci. 1983, 118, 329-344. (12) Brandt, E. S.Anal. Chem. 1985,57, 1276-1280.
2 -0.3 V
A
L I
IO00
I500
Raman Shift, cm-' Figure 1. SERS spectra of 0.050 M 4-pyridinecarboxaldehydein aqueous 0.10 M KCI using a silver electrode. Potentials are vs SCE. Insets show expanded view of carbonyl band near 1710 cm-I.
is not too negative. At these less negative potentials (-0.2to -0.4 V), 4-pyridinecarboxaldehyde does not completely hydrate upon adsorption as was suggested by Bunding and Bell. Allen and Van Duyne" have considered orientational effects on band intensities and have concluded that the relative intensities should be given by eq 1, where the intensity of the carbonyl band is expressed relative to the intense symmetrical ring-breathing mode near 1000 cm-', in both the S E R S and normal Raman solution spectra (NRS). 0 is the angle of the carbon-oxygen bond in the adsorbed species with respect to the surface normal. For vertical orientation of 4-pyridinecarboxaldehyde(adsorption via the ring nitrogen atom), 0 will be 60'. Analysis of the SERS data a t -0.20V and N R S (0.10M KCI) gave 0 = 5 7 O , suggesting a vertical orientation at this potential. However, some process leads to the diminution of the carbonyl band intensity as the potential is made negative. Possibly the free aldehyde is reduced at the electrode, causing the disappearance of the band at 1710 cm-I. However, cyclic voltammetry of 0.010 M 4-pyridinecarboxaldehyde at a silver electrode in 0.10M KCl reveals virtually no faradaic current at (13) Allen, C. S.; Van Duyne, R. P. Chem. Phys. Lett. 1979,63,455-459.
Anderson and Evans
6614 J . A m . Chem. SOC.,Vol. 110, No. 20, 1988 1. 30
A
3. 86
1
D
C
1
B
I
A
I
6. 88
h
. . . . . . . . . . . . . . . . . . . . .
1500 1600 1700 Raman Shift, cm-1
Raman Shift, cm-’ Figure 2. Normal Raman spectra of 4-pyridinecarboxaldehyde: (A) pure liquid; (B) 0.10 M aqueous solution; (C) 0.10 M in 0.50 M HCI;(D) 0.10 M in 0.50 M NaOH.
Figure 3. SERS spectra of 0.050 M 4-pyridinecarboxaldehydein various buffers of the pH indicated. All spectra were obtained at -0.20 V vs
potentials as negative as -0.8 V, making it highly unlikely that the adsorbed free aldehyde is being reduced at even the most negative potential used in the S E R S studies. As suggested by Bunding and Bell,lL hydration of the surface species may be responsible for the diminution of the carbonyl Raman band. To explore this possibility, it will be helpful to examine the acid-base and hydration equilibria of 4-pyridinecarboxaldehydein aqueous solutions. Spectroscopic Behavior of 4-Pyridinecarboxaldehyde as a Function of Solution pH. It has long been known that the extent of hydration of 4-pyridinecarboxaldehydedepends upon the solution pH. At p H
